Most diving helmets are rigid and waterproof and are connected to the diver's body or a diver's suit by a flexible tubular member known as a neck dam. The neck dam acts as the flexible waterproof connector between the rigid helmet and the diver's body or suit. In some cases, particularly with wet suits, the neck dam seals directly to the diver's neck. The tubular dam is connected by a water tight joint to the bottom edge of the rigid diver's helmet.
The usual breathing regulator maintains the breathing gas pressure within the diver's suit and helmet at the same pressure as the water depth at which the diver is working. When a diver is standing upright the neck dam maintains itself generally close to the diver's neck. However, when a diver leans forward the internal air pressure causes the dam to bulge outwardly. This increases the buoyancy of the neck dam-helmet combination and the helmet moves upwardly with respect to the diver's head, moving the helmet to an awkward position.
To counteract this added buoyancy, helmets have been made heavier. Also, attempts have been made to hold the neck dam against expansion, but these attempts have interfered with, the free motion of the helmet as the diver rotates his head.